Thanks for the great comments chaps, really appreciated.
Dave, I used Mr Color lacquers, 321 light brown and 517 brown. These are both a bit semi gloss so I added some of their "flat base rough" mixture, which adds a bit of texture and removes the sheen. I never check exactly how much I thin my paint, I just go until it feels right, but probably around 1 part of the paint & base mixture to at least 4 or 5 levelling thinner.
I dusted on the light brown first, then lowered the pressure right down until the paint was just spitting out of the
airbrush to add some splatters.
I then did the same with a 50/50 ish mix of the light and darker browns on a smaller area, then yet again with just the darker brown, working on an even smaller area, hoping to give the impression of wetter dust lower down and in the recesses.
Some dry pigment work might have added even more to the effect, but I tend to get myself in a mess with those so chose not to use them this time.
Hope that helps.